The present invention relates to a fluid pressure shock absorber such as a hydraulic shock absorber mounted on a suspension apparatus of a vehicle such as an automobile. In particular, the present invention relates to a fluid pressure shock absorber including a backpressure chamber for controlling a valve-opening pressure of a main valve which generates a damping force.
Generally, a cylindrical hydraulic shock absorber mounted on a suspension apparatus of a vehicle such as an automobile comprises a sealed cylinder in which oil is contained, a piston rod, a piston coupled to the piston rod and slidably fitted in the cylinder, and a damping force generating mechanism provided at a piston portion, the damping force generating mechanism comprising an orifice, a disk valve and the like. In the hydraulic shock absorber configured as above, an extension or compression movement of the piston rod causes a sliding movement of the piston in the cylinder which in turn causes an oil flow, and a damping force is generated by controlling the thus-caused oil flow with use of the orifice and the disk valve. In a low piston speed range, a damping force of orifice characteristics is generated by the orifice, and in a high piston speed range, a damping force of valve characteristics is generated by the disk valve that is bent and opened.
This conventional hydraulic shock absorber has a disadvantage in that it is difficult to flexibly set damping force characteristics because in a low piston speed range a damping force depends on an area of an orifice flow passage and in the high piston speed range a damping force depends on a predetermined valve-opening pressure of the disk valve; that is, a sufficiently large damping force cannot be obtained in the high piston speed range if damping force characteristics is arranged such that a small damping force is obtained in the low piston speed range, and a damping force becomes excessively large in the low piston speed range if damping force characteristics are arranged such that a large damping force is obtained in the high piston speed range.
As an improved art, for example, Japanese Patent Application Public Disclosure No. 2005-344911 discloses a hydraulic shock absorber comprising backpressure chambers provided on back surface sides of disk valves. In this hydraulic shock absorber, an amount part of oil is introduced in the backpressure chamber so that a pressure of the backpressure chamber acts on the disk valve in a valve-closing direction of the disk valve. A valve-opening pressure of the disk valve is controlled by this mechanism, whereby it is possible to improve flexibility in setting damping force characteristics.
The hydraulic shock absorber disclosed in Japanese Patent Application Public Disclosure No. 2005-344911 further comprises check valves through which the extension-side and compression-side backpressure chambers are respectively in communication with cylinder chambers on downstream sides of the backpressure chambers. During an extension stroke of a piston rod, an increased pressure of the cylinder chamber is introduced into the compression-side backpressure chamber through the check valve, thereby preventing valve opening of the compression-side main valve. During a compression stroke of the piston rod, an increased pressure of the cylinder chamber is introduced into the extension-side backpressure chamber through the check valve, thereby preventing valve opening of the extension-side main valve. Accordingly, it is possible to generate a stable damping force.